Rapid onset of action (< 30s, peak at 5min) due to lipid solubility (600x that of morphine).

90x more lipid soluble than morphine, but more rapid onset than fentanyl. This is due to: 1. Low pKa means a greater proportion is unionised at physiological pH. 2. Lower potency of alfentanil compared to fentanyl means a greater dose is required (Bowman's Principle)